During the 2004 elections, I recall that something like 50% of the public thought that the media favored John Kerry, 20% thought it favored Bush, and 30% thought it was balanced.
In the 2008 elections, 70% think that the media favors Obama, 10% think it favors McCain, and 20% thinks it is balanced.
Grains of salt time. I'm too lazy to run these down, so I may have the numbers somewhat wrong. They are approximate anyway, though I think they're close. I would remember if the numbers came from some obviously partisan source in either direction, so I conclude they come from groups I know little of or believe to be relatively reliable. I may delude myself there. Further, I don't know if the way the questions were asked match up well enough to be a fair comparison. This might be apples and oranges - or apples and pears, anyway, which are more similar.
Most importantly, remember that just because more people think something doesn't make it true. Martians visiting and evaluating media bias might see things quite differently. (Or Venusians. Martians may be unreliable).
With all that salt, it can be fairly asked whether these numbers mean anything valuable at all. I find one thing interesting about them - they are quite different in just four years. Conservatives might find some long-term encouragement there.
And by the way, when do we get to start using the phrase "Media Bias Deniers?"